Penetrating ray transducer



Oct. 8, 1957 J c s "ET AL 2,809,323

PENETRATING RAY TRANSDUCER Filed March 24, 1954 THROUGH AMPLIFIER T0"rm: CONTROL emu JOHN E JACOBS HAROLD BERGER ATTORNEY OF THE PICTURETUBE INVENTORF- gwwa @mg gu johir' E. Jacobs, Milwaukee, and HaroldBerger, Wankeshn, Wish; assignors to GenerahEleeti-ic Company, acorporation of New York Appncationlyimcir 24, 19st, seriaiN'o. 418,414

' 5 claims- (charm-= The present invention r'e'lafesi'n. general toelectronics and has more particular reference to the conversion tooptically visible form of latentjimages carried in. penenoting rays suchas X -rays', the invention pertaining more especially to improved meansfor and method of? producing electrical impulses corresponding withlatent images carried by a beam of penetrating rays whereby to actuatepicture reproducing equipment of the sort commonly employed torproducing visible television pictures.

A latent invisible picture image ofi an object may be formed orentrained in a beam. of penetrating rays, such as I i-rays, gamma rays,or other invisible ray-like emanations of penetrating character, bycausing the rays to traverseorscan the obiect to be pictured.Sucha-latent ray carried picture imag'efmay "bereducedto visible form byphotographic procedures, as by applying the picture carrying gaybeamnponray sensitive film, as is common X-ray photography orradiography. It is also conventional, as in X-ray fluoroscopy, toconvert-a ray carried latent picture imageto visible form byapplying theray directly upon-a sensitive screen 'adapted to glow differentiallyinproportion to the intensity or rayrimpinget ment thereon, andhence toreproduce the latent my carried image as avisible fluorescent picture.on the ray sensitive-screen. t I g j l The foregoing procedures bothinvolve the application at the ray carrying beam through the objectbeingpictured anddirectly. upon the sensitive film or screenin which thelatentray carried picture is converted to visible form. ln order' tothnsproduce a visible picture of satisfactory quality; itis frequently,necessarytoemploy image carrying }'rays of relatively high intensityrepresenting a hazard to theoperator of the; picture producing equipment, especially where, as in X-ray fluoroscopy, the resulting visiblepicture must be viewedfrom a station closely adjacent the screen,Furthermore, where the object Bein pictured is a living thing, the'required ray quantafor optimumpict'nring purposes may be of dangero'uslyhighvaluegrepresenting a haz-aidto the thingbeing pictured. I

An -importantobject of the present invention resides in providing meansforanda method of produ'cing'visible picfureacorresponding withimageslatently' carried by penetrating rays, "such as X -rays, at aremoterpicturing station to Whichtheimage carrying rays manot'penetrate, therebyeliminating the hazard of penetratingray Z,8h9,323Patented Dot. 8, 1 957 2- pulses corresponding with a latent ray carriedpicture image and adapted for the operation of a picturing tube of thesort commonly employed for television picture reproddcin purposes.

Another important object resides in. providing. for the directconversion of penetrating ray energy to electrical ener y; a furtherobject being to accomplish such transformation Within a layer ofsubstantially uniform thickness and; comprising a ray sensitivesemi-conductor; a further object being to support the energy conversion'lay'riipon a suitable plate or panel of electrical conducting,material, whereby latent image carrying rays applied to the conversionlayer may be converted to corresponding electrical impulses byprogressively scanning the layer as by means of an electron beam ofpencil-like character.

Another important object is to produce electrical impnl'sescorresponding with latent ray carried images by applying imag carryingrays upon a layer 'of ray sensitive material havin impedancecharacteristics which fluctuate precisely in accordance with theintensity of image carrying rays impinging thereon, and by providing"for the measurement of impedance variations in each integral portion ofthe layer in terms of electrical current flow therein, whereby to roduceelectrical impulses cor- V responding with the degree of ray excitationof the layer,

and hence with the latent picture image carried by the layer excitingrays.

Another im ortant object is to provide for electroniealty scannin thelayer with a pencil-like scanning beam or electrons in order thus tomeasure, progressiv ly and nco'es'sively', in rapidly repetitivefashion, the momentary r impedanc characteristics of each integralportion of the layer, to thereby develop electrical impulses, in acircuit associat d withthe scanning beam and the layer, the impulses sodeveloped representing measurements ofthe omentary impedance values ofthe integral portions of the layer, and hence corresponding with thelatent image tram the plate remote side of the layer, wherebyapplication of image carrying rays on said layer will reduce momentarilythe voltage across the layer at the place of my impin ement so that theintensity of said ray may be measured in. terms of momentary currentflow between said grid and said layerlrequired to restore the potentialat said placeof ray impingement; a further object being to accomplishpotential restoration at a zone of ray impingement' in response to thescanning of said zone by a scanningbeam cyclically traversing theconversion layer through said biasing screen, to thereby produce voltagefluctuations, in timed relation with respect to the movemerit of thescanning beam and corresponding with the voltage restorationsaccomplished in the successively scanned zones of said layer.

A further object of the invention is to form the ray sensitiveconversion layer of crystalline semi-conductive material in finelydivided condition; afurther object being to employ cadmium sulphide,zinc sulphide, mercury sulphide, cadmium selenide, or any ra'yresponsive semiconductor having a dark resistance in the range of LO-e10 ohms per cubic centimeter.

The foregoing and numerous other important objects,

from the following description, which, taken in connecwith theaccompanying drawings, discloses a pre- 'f'e'r'r'ed embodiment of theinvention.

Referrin to the drawings: Fig. 1 is a diagrammatic view showingapparatus for producing visible images in accordance with the teachingsof the present invention;

Fig. 2 is an enlarged sectional view taken through a penetrating raytransducer embodying the present invention and forming part of theapparatus shown in Fig. l; and

Figs. 3, 4 and are sectional views taken substantially along the lines33, 4-4 and 5-5 in Fig. 2.

To illustrate the invention the drawings show picture producingapparatus comprising a penetrating ray transducer adapted to receivepenetrating rays 12, such as X-rays, emanating from any suitable raysource and latently carrying the image of an object 13 disposed in thepath of the rays 12, between the ray source and the transducer 11. Thetransducer is adapted to produce electrical impulses corresponding withthe characteristics of the image of the object 13 latently carried bythe rays 12. The transducer accordingly may be interconnected with asuitable translation system 14 for transmitting electrical impulses toand applying the same for the operation of a picture reproducing tube 15of the sort commonly employed for television picture producing purposes,whereby the picture reproducing tube 15 may be operated for thereproduction of a visible picture corresponding with the latent pictureimage carried by the rays 12 and applied to the transducer 11.

The tube 15 may comprise a sealed and evacuated envelope having anenlarged end provided with a wall 16 of transparent material, such asglass, and embodying a picture producing screen 16 of fluorescentmaterial adapted to glow in response to electron impingement thereon,the envelope, remote from and opposite the end wall 16, forming a barrel17 containing a suitable electron gun structure 17' for generating andapplying a pencil-like scanning beam 18 upon the picture producingscreen at the Wall 16. Associated beam controlling and deflecting meansof any suitable or preferred character, including electrostaticdeflecting plates within the envelope or focusing and deflecting coilsdisposed outwardly of the envelope, may be provided for directing thescanning beam 18 to progressively and successively traverse thefluorescent screen, at the end wall 16, in accordance with a selectedscanning pattern, to thereby excite the screen to glow differentially inaccordance with the fluctuating intensity of the scanning beam. Thefluorescent material of the screen on the wall 16 is adapted to glow,atthe place of impingement of the beam 18, in accordance with thefluctuatin intensity of said beam. The fluorescent screen accordinglywill produce a picture visible outwardly of the tube through thetransparent end wall 16. The production of such picture is, of course,caused by fluctuating electrical impulses applied to the electron gun 17for the intensity control of the electron beam 18 in timed relation withrespect to the screen scanning movement of the beam. Fluctuatingelectrical impulses for the intensity control of the electron beam 18and also for causing the scanning movement of the beam, as

shown in Fig. 1, may be delivered through the translation system 14 inaccordance with well known television picture reproducing procedures.

In order to develop electrical impulses for the operation of the picturereproducing tube 15 to produce a visible picture corresponding with thelatent image carried by the rays 12, the transducer 11, as shown moreparticularly in Fig. 2, may comprise a sealed and evacuated envelope 21,preferably formed of glass to provide a tubular electron gun housing 22of limited sectional size, at one end of the envelope, and an enlargedportion 23 forming a scanning chamber in open communication with the gunhousing at one end thereof. The envelope 21 may include a preferablyoutwardly bowed end wall 24 sealing and enclosing the end of thescanning chamber remote from the gun housing. Latent image pick-up means25 may be provided immediately within the end wall 24 in positiondisposed in the path of the image carrying rays 12, the pick-up meansbeing also disposed in alinement with the gun housing in position to bescanned by an electron beam 26 emitted by an electron beam gun structure27 mounted within the housing 22.

The gun structure 27 may be of any suitable, preferred or convenientform and may be used in association with conventional horizontal andvertical electrostatic beam deflection plates 28; or magnetic beamdeflecting coils disposed outwardly of the gun housing may be used inconjunction with the gun for beam deflecting purposes, in accordancewith well known electron scanning beam operating technique. The imagepick-up means 25, as shown, comprises a relatively thin layer of raysensitive electrical semi-conducting material S, said layer havingsubstantially uniform thickness and being coated upon a suitable supportmember which may conveniently comprise a panel or plate 29 of electricalconducting material.

For the purpose of the present disclosure, a semi-conductor may bedefined as a material having electrical resistance, or reactance, orboth, which vary in accordance with the intensity of rays to which thesubstance is exposed. Electrical resistance and reactance, eitherinductive or capacitive reactance, or both, are those characteristics ofelectrical conductors which tend to prevent the flow of electricalcurrent therethrough under the influence of an electromotive force. Thecombined flow resistive effect of resistance and reactance in aconductor material is commonly referred to as the electrical impedanceof the material.

In the absence of rays to which it is responsive, a semiconductor mayhave impedance characteristics of such high order as to constitute thematerial as a virtual insulator capable of substantially preventing theflow of electrical current therethrough. When irradiated with rays towhich it is responsive, the impedance of the semiconductor material maybe reduced as a proportional function of incident ray intensity, so thatthe material becomes electrically conducting in proportion to theintensity of exciting rays impinging thereon. As a consequence, such rayinduced or controlled change in the impedance of the semi-conductor maybe measured to determine the intensity of rays impinging on thesemi-conductor.

In accordance with the present invention, the ray sensitive materialforming the layer S preferably comprises a crystalline semi-conductormaterial, such as cadmium sulphide or the combined sulphides of zinc andcadmium, rendered sensitive to X-rays by treatment with a metallicactivator, such as silver, activation of the crystalline semiconductormaterial being accomplished by the dispersion of minute atomicquantities of the activator throughout the lattice structure of thecrystalline material, as taught in the copending application for UnitedStates Letters Patent Serial No. 269,276, filed January 31, 1952,covering the invention of John E. Jacobs in Method of and Apparatus forProducing Synthetic Crystals and Resulting Product.

. Irradiation of the layer S by penetrating rays 12 carrying the-latentimage of the object 13 will correspondingly alter the impedance of thecrystalilne material of the layer and thus apply a latent image of theobject in the layer S, said latent image being defined in terms of theimpedance characteristics of each integral portion of the layer. Thepresentinvention contemplates means for successively and repetitivelymeasuring the ray determined impedance of each integral portion of thelayer S in order to produce a transmissible signal corresponding withthe impedance defined latent image in the layer S. To that end, thepick-up means 25, in addition to the layer S and its supporting panel29, may comprise a foraminous layer 31 mounted in closely spacedrelationship with respect to the panel 29 in position overlying thesurface thereof which carries the layer S.

' The layer 29 may conveniently comprise a flat relatively g 'thiripanel of aluminum, copper,- or other preferred electrieal conductingmaterial eap'ahle of sup orting the layer S, while the foraminous layer30 may comprise a fine mesh woven metal wire scre'n, or the order ofsixty meshes per linealinch, the layers 29 and 30 being electricallyinsulated the one frorn'the other, and being supported in spaced apartposition, the spac'ement of the layers being of the order ofone-eighthineh. To this end,

' the foraminous layer 30 may be tantly stretchd and setions extendingoutwardly of the envelope through the re- .entrant portions 33, saidoutwardly extending portions being electrically connected, as at 36,with preferably flexible conductors 37. The frame 31 and the foraminouslayer 30 supported thereon, as well as the layer 29 may be mounted uponthe stems '34 in manner presenting the layers 29 and 30 immediatelyinwardly of the end wall 24 of the envelope, in position centered withrespect to the electron gun structure 27, the foraminous layer 30 beingdisposed in parallel spaced relation with respect to the layer 29 andbetween said layer 29 and the electron gun structure.

As shown more especially in Figs, 3 and 5, the panel 29 and the frame 31may be mounted upon the stems 34 infashion electrically connecting thepanel with one of the stems, while electrically connecting the frame 31and the foraminous layer 30 carried thereby with another of said stems34. Accordingly, as shown in Fig. 3, one of the stems 34 may carry aninsulating collar 38 adapted to :fit within and extend through anopening formed in the frame 31. The stem may also carryan insulatingwasher .39 extending on the stern, between the frame 31 and the panel29, which is also formed with an opening there- :through for receivingthe stem 34. The panel 29, frame .31, and insulating members 38 and 39may be secured on the stem 34 by and between suitable clamping members40 and 41, a washer 42 of electrical conducting mate- :rial beingdisposed between the panel 29 and the clamping Linember 41. V A t Thestructure shown in Fig. 3 thus mechanically mounts the panel 29 andforaminous layer 30 on the stem 34, andelectrically connects the panelwith the stern, while insulating the layer 30 and its supporting frame31 from thepanel and the supporting stem.

As shown Fig. 5, one of the stems 34 may carry a mountin collar 38' ofelectrical conducting material adapted to snugly fit within an openingformed through the frame 31, in order to electrically connect the frame:v'vith the stem. The stem may also carry an insulating coll'a'r 39'having a portion adapted to extend through an opening formed in thepanel 29 in order to space and insulate the panel from the frame 31, andto insulate the panel from the stern. The p'a'nel 29, frame 31, and thememberss's' and 39' ma be secured on the stem 34 by and between suitableclamping members 40 and 41,

an insulating washer 42' being disposed between the panel 29 and theclamping member 41.

The structure shown in Fig. thus mechanically mounts the panel 29 andforaminous layer 39 on the stern '34, and electrically connects theforaminons layer with the stem, while insulating thepanel 29 fromthe'forafnin'ous layer and the supporting stern.

When the transducer is in operation, a potential diirerence of desiredvalue, supplied from a suitable unidirectional power source 43, may bemaintained between the plate 29 and the foraminous mesh layer 30. Tothis end,-the power sonree ts-may be mediall connected to ground, thepositive and negative ends or the source being connected across apotentiometer forming "resistor '44, havin an associated adjustable ta'pconnected with the mesh layer 30",- as through the conductor 37, andpreferably through a resistor 45. By adjusting the tap of thepotentiometer any desired positive or negative voltage with respect toground, within the range of volta e afforded by the "source 43, may beapplied to the mesh layer 30. If desired, the mesh layer may beeonnected to ground through a'condenser 46. The plate 29 may beconnected to ground, as by way of the'e'onductor 37 and an outputresistor 47.

When the layer S is dark, that is to say when it is'not being irradiatedby rays to which it is sensitively responsive, the voltage drop acrossthe layer, when the same is scanned, will be merely the relatively smalldark current loss characteristic of the semi-conductor material of whichthe layers is formed. When the layer is irradiated by rays 12 to whichit is sensitive, the voltage drop across the layer, at the scanningspot, will be directly proportional to the intensity of layer impingingrays at the scanning spot. Such loss is instantly restored by the powersource 43, since the scanning beam functions as a conduction pathbetween the foraminous layer 30 and the facing surface of the layer S.The extent of such restoration may be measured in terms of voltageacross the output resistor 47. Voltage thus developed across theresistor 47 will accurately define the latent image imposed by excitingrays 12 in the layer S, and consequently means may be provided forcontrolling the op eration of the picture tube 15 in accordance with.the 7 Voltage developed across the resistor 47, as by applying saidvoltage through a condenser 48' to any preferred amplifying system 49,for application on the control grid of the electron gun of the viewingtube, to control the picture reproducing intensity of the electron beam18.

It will be seen from the foregoing that thepresent invention in itsbroader aspect contemplates the direct conversion of X-ray energy tocorresponding electrical impulses directly in the semi-conductive layerS. When the layer S is scanned by the electron beam, the variations inenergy absorbed by the layer from the'beam, in order to restore rayinduced energy losses in the layer, are used to measure and henceindicate the X-ray pattern applied to the layer S.

Alteration or adjustment of the operating potential applied on theforaminous layer 30 may be utilized to control the rate of response ofthe transducer to variations in the intensity of the exciting rays 12.If, for example, the device be operated at a positive potential of theorder of fifty volts, the image response is substantially instantaneous,that is to say, the layer S retains substantially no electrical evidenceof an image following the scanning of the layer by the beam 26. If,however, the positive potential applied on the foraminous layer 30 israised to a value of the order of two hundred to three hundred volts,the latent image imposed in the layers, at the instant when thepotential is thus raised, is effectively locked in the layer and may bedetected by electronically scanning the layer for extended periods, ofthe order of several minutes, before the so locked image fades out to anextent such that it can no longer be detected. The latent image, whichat the instant of locking is present in the layer, in the form ofunneutralized positive charges distributed throughout the layer, becomeslocked in the layer in the form of electron deficiencies, throughout thelayer, so that continued scanning will continue to show the latentimage. Neither continuation nor elimination of the impingement of layerexciting rays 12 affects the intensity of a latent image so locked inthe layer S by a sudden increase in positive mesh potential.

When a system of the sort described is employed for X-ray fluoroscopy,the equipment may normally be operated under low positive voltageexcitation of the fo- I raminous layer 30, in order that there will besubstantially no image retention in the layer S. Whenever thefluoroscopic examination reveals a condition of unusual interest in theobserved image of an examination object, the said image may instantly belocked in the layer S and scanned to apply a visible reproductionthereof on the screen of the viewing tube 15, and said visiblereproduction of the so locked image may then be photographed forpermanent record purposes, or may be studied during the extendedinterval within which the locked image gradually fades. Obviously theapparatus of the present invention may be employed to eliminateconventional X- ray film and darkroom practice in connection withradiography.

Apparatus embodying the present invention also may be utilized in placeof so-called spot film devices, of the sort described, for example, inLetters Patent of the United States No. 2,552,858, issued May 15, 1951,on the invention of Robert I. Mueller and Ivan Burgeson inSerialographic Apparatus. For such service, X-ray fluoroscopy may beconducted by adjusting the pick-up tube or transducer 11 for fastresponse. When a condition in the examination object is observed uponthe screen of the viewing tube 15, of which a permanent record isdesired, the image may be locked in the layer S and X-radiationimmediately discontinued. The locked image may then be reproduced on thescreen of the viewing tube, and there photographed. For such service,the present invention provides manifest advantages, in that the patientneed not be exposed to additional radiation in order to obtain a desiredpicture Furthermore, there is no delay between observation of acondition requiring photographic recordation and the locking of theimage for photography in due course and at leisure. Moreover, thepresent invention eliminates all mechanical problems associated with theprojection and retraction of film carrying cassettes into picture makingposition in spot film apparatus of the sort illustrated and described inLetters Patent of the United States No. 2,552,858.

It has been determined that the power to retain a ray induced imagevaries as a characteristic of the particular ray sensitivesemi-conductor material employed in the layer S. Where a semi-conductormaterial such as cadmium sulphide, containing copper as an activator, isL,

employed, the image response is relatively slow; indeed, where copper isemployed as an activating medium, even when the device is operated atvoltages in the foraminous layer Stl such that image persistence is at aminimum, the

persistence of the ray induced image is such that the device is oflittle use for the observation of moving objects. The image locked inphotoconductors of this type, either because of the inherent slowresponse characteristic of the photoconductor, or by positive potentialincrease on the layer 39 of a device embodying fast image response raysensitive material, may be rapidly wiped out of a sensitive layer Scontaining the same by momentarily reducing the potential on theforaminous layer 39 to a negative value, of the order of several hundredvolts. Such voltage reduction may be accomplished at relatively highfrequency and automatically by employing a control wave of square form,which may be derived from the vertical scan synchronizing generator. Bythus providing for applying image wipe-oil? negative voltageperiodically upon the layer 30 at relatively high frequency, theeffective results of a layer having fast response characteristics may beobtained even where the layer comprises material having substantiallyslow image characteristics.

A transducer 11 embodying a ray sensitive layer S, of the persistantimage or slow image response type, may be applied to great advantage inthe radiography of stationary objects. The image storing ability of theslow response material allows substantial reduction in the intensity ofpicturing rays, for the reason that the sensitive material has acumulative action under ray impingement, similar to that possessed byphotographic film under the influence of light; and the image erasingfeature is, of course, of considerable advantage in that it allows thelayer S, in effect, to be wiped clean of an image, as soon as the sameis no longer wanted.

It is thought that the invention and its numerous attendant advantageswill be fully understood from the foregoing description, and it isobvious that numerous changes may be made in the form, construction andarrangement of the several parts without departing from the spirit orscope of the invention, or sacrificing any of its attendant advantages,the form herein disclosed being a preferred embodiment for the purposeof illustrating the invention.

The invention is hereby claimed as follows:

1. The method of converting the persistence characteristics of a layerof semi-conductor material employed in a photo-conductive transducer forconversion of an X-ray image to a modulated electrical signal whenscanned with an electron beam, the tube being of the type wherein a meshtype grid element is maintained in spaced relation to the layer andintermediate the beam source and the layer, said method consisting ofapplying to said grid a positive potential in the order of volts toeffect a condition of minimum persistence, and then applying a positivepotential in the order of 200 or more volts to effect a condition ofmaximum persistence.

2. The method of controlling the persistence characteristics of a layerof semi-conductor material in a tube of the type described consisting ofproviding direct current sources in the order of 50 volts and 200 to 300volts, respectively, and applying one and then the other of said sourcesto the grid-mesh, substantially to change the persistence characteristicof said layer.

3. The method of controlling the persistence characteristics ofsemi-conductor material mounted in spaced relation to a grid-mesh in atube of the character described consisting of maintaining the mesh at afixed positive potential level to effect one characteristic and thensuddenly subjecting the mesh to a different positive potential level toobtain a substantially different characteristic.

4. The method of establishing either a low or a high persistencecharacteristic in a layer of cadmium or zinc sulphide or combinationsthereof in a grid-mesh controlled photo-conductive transducer of thetype described, consisting of applying a direct current potential in theorder of 50 volts to the grid to establish a low persistencecharacteristic and then manually switching the grid to a potential levelin the order of 200 or more volts direct current to establish a highpersistence characteristic.

5. The method of controlling the persistence characteristic of anelement of cadmium or zinc sulphide or combinations thereof employed ina grid controlled cathode ray tube as a transducer for conversion ofX-rays to an electrical signal, consisting of maintaining the grid at adirect current potential in the order of 50 volts to effect persistencecharacteristics of less than one second and then changing the potentiallevel of said grid to the order of 200 or more direct current volts toeffect locking of the persistence characteristics for a period of two ormore minutes.

References Cited in the file of this patent UNITED STATES PATENTS2,518,434 Lubszynski Aug. 8, 1950 2,544,754 Townes Mar. 13, 19512,622,219 Schagen Dec. 16, 1952 2,747,131 Sheldon May 22, 1956

